An HBT produces a current output between its emitter and its collector. The output current (IC) is some function of the input current (IB) imposed between its base and its collector. Of particular interest is the case where the output is a good linear approximation of the input, so their ratio is well-described as a current gain factor a/k/a current gain approximated by the form IC=xcex2 IB. The input function may be time-varying, so a value fT is ordinarily defined to denote the frequency at which the current gain of the HBT falls to unity. fT can be estimated from:                               1                      2            ⁢            π            ⁢                          xe2x80x83                        ⁢                          f              T                                      =                              τ            F                    +                                                                      k                  B                                ⁢                T                                            qI                C                                      ⁢                          (                                                C                  E                                +                                  C                  BC                                            )                                +                                    (                                                R                  E                                +                                  R                  C                                            )                        ⁢                          C              BC                                                          (        1        )            
A related value fmax is defined to denote the frequency at which the power gain of the HBT falls to unity, which may be estimated from:                               f          max                =                                            f              T                                      8              ⁢              π              ⁢                              xe2x80x83                            ⁢                              R                B                            ⁢                              C                BC                                                                        (        2        )            
In equations (1) and (2), CE and CBC refer to the capacitances across the emitter-base and the base-collector junctions, respectively. RB, RE and RC refer to the extrinsic base, emitter and collector resistances, respectively. IC is the total collector current. kB is Boltzmann""s constant. T is the absolute temperature. q is the charge of an electron. xcfx84F is the emitter-to-collector transit time, which sums the base transit time (xcfx84B) and the collector transit time (xcfx84C), so xcfx84F=xcfx84B+xcfx84C.
High output power is commercially valuable. It is often achieved in part by using a high output voltage. A high voltage requires that the transistor""s breakdown voltage across the base, between the emitter and the collector, (commonly denoted in the literature as VBECO or BVECO) be as high, or conservatively twice as high, as the output voltage. The breakdown voltage is determined by the materials properties of the semiconductor used for the collector, and by the other design parameters, such as alloy composition and doping density. An important design goal is therefore to increase BVECO for a given fT, or equivalently, to increase fT for a given BVECO.
Exceeding BVECO can impair system performance. In some cases, the damage is irreversible to the transistor itself or to other elements of the circuit comprising the transistor. Yet requiring that a transistor operate at a voltage with some safety factor far below BVECO, requires a compromise in performance and/or using a more expensive, large, power-hungry transistor.
What is needed is a transistor which allows operation at a first, very high speed fT up to a first voltage level, followed by operation at a second, lower speed fT up to a second, higher voltage level. The first voltage level would be the normal operating regime of the transistor, providing a consistently high performance. The second voltage level would provide a safety margin, operating at a reduced performance level in over-voltage conditions.
In the preferred embodiment of the invention disclosed herein, such a transistor is designed with said second voltage level near the nominal breakdown voltage of the device, and said first voltage level at roughly half of said second voltage level.
More generally, the invention specifies a means for improving the performance and breakdown voltage of an HBT, wherein the means entails a novel base-collector structure. The rest of the transistor will preferably be optimized to make good use of the novel collector, though such optimizations are not strictly necessary to enable the invention.